1. Field of Invention
This invention relates to projection displays, and specifically to illumination engines for projection displays.
2. Description of the Related Art
There are several major kinds of display. These displays vary significantly both in the amount of space they require and their cost. The most common type of display is the cathode ray tube (C RT) display. CRTs are inexpensive and bright, but require a large amount of space. Another common kind of display is the direct view liquid crystal display (LCD) panel. Although LCD panels with small display areas are relatively inexpensive, those with larger display areas may cost several times as much as a comparable CRT display. As a result, LCD panels are not as popular as CRTs, unless space is at a premium. Thus, LCD panels are often used in crowded areas, e.g. restaurant cashier counters. It would be desirable for an image projection system to be compact and inexpensive.
For larger displays, two types of displays are most prominent: plasma display panels and projection displays. Plasma display panels are thin, and thus occupy only a small amount of space. Their resolution, however, is not as high as that of a comparable projection display. In addition, plasma displays are quite expensive. Plasma display panels are therefore not as popular as the other types of displays. It would be desirable for an image projection system to have high resolution and be inexpensive.
Projection displays work by shining light from an illumination engine, such as a white light from an arc or a filament lamp, onto an imager, such as a digital micro-mirror device (DMD), an LCD, or an liquid crystal on silicon (LCOS) chip. The imager may be modulated with an image signal to control its reflection and transmission properties. The imager may respond to the image signal by either reflecting or transmitting the light to create an image.
Much work has been done on projection displays using small display imagers such as DMD displays, LCDs, and LCOS displays. Such displays, however, require expensive illumination engines. While these displays provide advantages for displays with large screens, these displays are not used as widely in displays with small screens, due to the high cost of the illumination engine. Therefore, there exists a need for a compact and low cost illumination engine that can be applied to smaller displays, such as those of 10″ to 35″. These displays could be used in, e.g. computer monitors and small televisions since they will occupy a small amount of space. With the advancement in the LED technologies, future high lumen output LEDs can have a potential of illuminating a large screen television in the 60″ range.
A color signal may be fed into the imager of the projection system in synchronism with the colors incident on the imager such that the output picture on the screen will be sequentially illuminated with the three colors. The eye retains the colors, merging the colors and giving an impression of an overall color picture.
Arc lamps and filament lamps, which are traditionally used as sources of radiation in such systems, have relatively short life spans. A light emitting diode (LED), in contrast, may have a lifetime of 100,000 hours, which is 20 to 50 times longer than an arc lamp. It would be desirable to be able to use an LED, or an array of LEDs, as a source of illumination in an image projection system.
FIG. 1 shows an LED 1 light source for use with an embodiment of the invention. An LED is an example of a device that produces light by electro-luminescence. An LED may be, e.g., a forward biased p-n junction. When an electric current is applied to the LED, minority carriers are injected into regions of the crystal where they can recombine with majority carriers, such as in the transition region and in the neutral regions near the p-n junction. The carriers emit radiation upon recombination. In materials characterized by direct recombination, such as e.g. Zinc Sulfide (ZnS), Gallium Arsenide (GaAs), Indium Antimony (InSb), Gallium Phosphorus (GaP), and, Gallium Nitride (GaN), the radiation may include a significant portion of visible light. This effect may be termed injection electro-luminescence.
The LED 1 may be mounted on a substrate 2, which may be insulating and a good conductor of heat, such as a Beryllium Oxide (BeO) substrate. Metal tracks 3, 5 or rails on top of the substrate 2 provide an electrical connection between the LED 1 and the other parts of the circuit. An LED 1 may have electrodes on the top and the bottom. An LED 1 may be soldered to one of the metal tracks 3 on the substrate, which forms one contact. The other contact is formed by wire bonding 4 the electrode on top of the LED 1 to another metal track 5 on the substrate. In the alternative, solder bumps may be used for soldering to the substrate instead of wire bonding. When an electrical current is applied to the LED 1, radiation is emitted.
An LED will generally emit radiation having a relatively narrow range of bandwidths due to the intrinsic properties of the LED materials. As a result, the output of the LEDs will normally be colored. LEDs that emit radiation in all of the colors from infrared to ultraviolet are readily available. Color LEDs are customarily used for indicators, while white LEDs are often used for general illumination. One of the common usages for color LEDs is for traffic lights.
The light source in a projection system should have a small etendue. As a result, a good collection and condensing system is required to collect the light from the light source and condense the light into the target. In the case of LEDs, most LEDs are packed into epoxy lenses, which increase the etendue of the LED.
On the other hand, if white LEDs are used, the output is directly compatible to an arc lamp illumination system, but the increase in the size of the emission area due the application of phosphor may increase the etendue, thus reducing brightness. Either or both of these schemes can be used depending on the system requirements.
Radiation of different colors produced by several LEDs may be combined to produce other colors or a net white output. The output of several LEDs may be combined by mixing their output in an homogenizer. In the alternative, a lens formed of clear epoxy 6 and a thin layer of white phosphor 7 may be applied to the top of an LED 1 which produces blue or UV radiation to ‘whiten’ the radiation, as shown in FIG. 2. In another embodiment, clear epoxy 6 may be replaced by white phosphor. For a sequential color projection system, each color may be turned on in sequence such that their outputs are synchronized with an imager in a projection system. This will produce an overall color display.
The radiation output from a plurality of LEDs may be combined to illuminate a screen. It is estimated that 10 to 30 LEDs of the types available in the market today would be needed to illuminate a screen on the order of 10″ to 21″, depending on the output intensity of the LEDs. As the output of the LEDs increases with the advancement of the technology, fewer LEDs will be needed. The total output etendue of the LEDs should match the etendue of the imager chip. For example, a 0.25 mm2 chip emitting in a hemisphere has an etendue of approximately E=0.25. For a 0.5″ imager chip at F/3.0, the etendue is approximately E=6.75. Thus, if there is no loss of etendue from the LEDs to the imager, a total of 6.75/0.25=27 LEDs can be used.
At present, although LEDs with output of over 100 lumens has been reported, the average output from a commonly available LED is about 20 lumens. Twenty-seven LEDs would thus produce a total of about 540 lumens. This would be sufficient for smaller displays, even after considering the loss budget of various components. LEDs may be expected to produce more lumens as the technology advances.
The light incident on the imager may be, e.g. filtered to produce a color image. Three primary colors, such as, e.g. red, green, and blue, may be fed to the imager by, e.g. filtering the light incident on the imager with, e.g. a rotating color wheel. Rotating color wheels are comprised of, e.g. red, green, and blue filters arranged about an axis and caused to rotate in the light. As each of the filters intersects the light incident on the imager, two of the colors will be filtered out while the third is transmitted. Rotating color wheels are simple and inexpensive, but incur losses due to the filtering. Furthermore, they require space for the motor. It would be desirable for a compact source of radiation to produce colored light with relatively low filtering losses.